This application is for a Patient-Oriented Research Career Development Award (K23) for Dr. Judith Tsui, an Assistant Professor in General Internal Medicine at Boston University School of Medicine, who is establishing herself as an investigator in hepatitis C virus (HCV) related outcomes. The research plan both extends her prior work in HCV, and allows her to grow in a new direction as a pain researcher focusing on substance users. The award will provide Dr. Tsui with the support necessary to accomplish the following goals: 1) develop expertise in the etiology of chronic pain in substance users with chronic viral infections like HCV and HIV; 2) publish research that will lead to improved treatments for pain among substance users; and 3) transition into an independently funded researcher by obtaining an R01. The comprehensive training plan includes formal coursework, as well as hands-on training in pain measurement and experimental pain models. Dr. Tsui's primary mentor will be Dr. Jeffrey Samet, an established investigator in the fields of substance use and chronic viral infections. She will receive additional mentoring from Dr. Jianren Mao, a leading translational pain researcher, and Dr. Robert Edwards, an accomplished junior investigator and clinical psychologist. Pain is a major problem among substance users, and a better understanding of its etiology is needed to improve clinical care. There is limited research to determine how viral infections such as HCV and HIV impact risk for developing chronic pain and painful conditions. Dr. Tsui will explore the novel hypotheses that HCV is associated with: 1) hypersensitivity to pain under experimental conditions and 2) clinical pain. To explore these hypotheses, Dr. Tsui will conduct an experimental, cross-sectional study of HCV mono- infected, HIV/HCV co-infected, and uninfected opioid addicts that compares experimental pain responses and self-reported clinical pain. She will assemble a cohort of 120 opioid dependent patients on buprenorphine or methadone: 40 HCV+/HIV-, 40 HCV+/HIV+ and 40 HCV-/HIV-controls. Participants will undergo the following experimental testing: 1) the cold-pressor test to assess cold pain tolerance 2) mechanical stimulus to test mechanical pain thresholds and 3) repetitive pinprick to assess for temporal summation. The study will also collect data on pain and pain-related variables using validated scales such as the Brief Pain Inventory, as well as measurements of pro- and anti-inflammatory cytokines (TNF-1, IL-2, IL-6, IL-2, IL-10 and IL-4) from peripheral blood samples. She will perform appropriate statistical testing to assess whether opioid addicts who are HCV infected (HCV+ alone or HCV+/HIV+) have different experimental pain responses (lower pain tolerance/thresholds and more temporal summation) and a higher prevalence of self-reported chronic pain, and to explore whether inflammatory cytokines mediate found associations between HCV and pain. This research will contribute to our understanding of the causes of pain among substance users, and will shape future studies to test new approaches (possibly including anti-HCV treatment) to prevent and treat pain in this vulnerable patient population.